1. Field of the Invention
This invention relates to a device for forming a compound oxide superconducting thin film. Specifically, the invention relates to a device for forming a compound oxide superconducting thin film by laser evaporation.
2. Prior Art
Superconductivity phenomenon, which is said to be related to a phase transition of electrons, has been recognized as a phenomenon observed only at very low temperatures whose realization requires liquid helium. But in 1986 Bednorz, Muller et al. discovered (La,Ba).sub.2 CuO.sub.4, which exhibits superconductivity at 30K. In 1987 Chu et al. discovered that YBa.sub.2 Cu.sub.3 O.sub.y has a superconductivity critical temperature Tc in the order of 90K. In 1988 Maeda et al. discovered the so-called Bi-system compound oxide superconducting material having a critical temperature higher than 100K. These compound oxide superconducting materials can exhibit superconductivity phenomenon even with cooling by inexpensive liquid nitrogen. The potential of the practical applicability of superconduction technology has been noted.
Compound oxide superconducting materials having the above-described high critical temperatures were originally prepared in sintered bodies by powder metallurgy. But the sintered bodies could not exhibit preferable characteristics, especially critical current density, etc. Recently processes for forming superconducting materials in thin films have been studied. Usually compound oxide superconducting thin films are formed on SrTiO.sub.3 mono crystal substrates, MgO mono crystal substrates, etc., by various kinds of evaporation, such as vacuum evaporation, spattering, etc.
The compound oxide thin film formed by various kinds of conventionally known evaporation techniques do not generally exhibit sufficient superconductivity as formed. To make them useful superconducting thin films, it is necessary to post-anneal the thus-formed compound oxide thin films. That is, it is generally known that compound oxide superconducting materials exhibit high oxygen non-stoichiometry, and also that the lower non-stoichiometry they have, the higher the superconductivity they exhibit. In conventional superconducting thin films, the post-annealing is performed for the purpose of supplementing oxygen due to the oxygen non-stoichiometry.
But it is known that when a thin film formed on a substrate is annealed, the substrate material is diffused into the thin film, and in the region near the substrate, the quality of the superconducting thin film is much degraded. The thus-treated thin film is usable only at a portion near the surface for experimental purposes, but it is not usable for practical purposes, such as production of various devices, etc. Under these circumstances, laser evaporation, which has not been so far noted, is suddenly noted.
Laser evaporation comprises irradiating a target with a high power laser beam, evaporating material from the target, and depositing the same on a substrate. The heating of the target and the substrate can be controlled independently of each other. It is not always necessary to make the ambient atmosphere highly vacuum. It is possible to increase the film forming speed by selecting suitable conditions. In addition, when a compound target is used, little composition change is observed between the composition of the target and that of the thin film which is formed. Thus, laser evaporation is considered suitable to form compound oxide superconducting materials in thin films. It has been confirmed that the compound oxide superconducting films formed by laser evaporation without the above-described post-annealing step exhibit useful superconductive characteristics. Laser evaporation is expected to be dominant in processes for forming oxide superconducting films. In the above-described laser evaporation process, when a laser beam irradiates a target, a blaze-like thing is generated. This blaze-like thing is called a plume. This is a collection of active materials generated from the surface of the target by the irradiation of the laser beam, and it is deposited on a substrate, so that a thin film is formed.
But a sectional area to be contributed to the film formation by the plume is generally smaller than the area of the substrate. A thin film is formed by laser evaporation only on a part of the film forming surface of the substrate. On a typically substrate having a size of about 20 mm.times.20 mm, a thin film which can exhibit effective superconductive characteristics is formed in about a 10 mm-diameter at most. Such small thin films are not practical.